Electric induction heating can be used to heat electrically conductive materials (for example, cast irons and steels) to temperatures in the austenitic range. The heated material is then quenched to temperatures where low transformation products, such as martensite and/or bainite are formed. There are two basic approaches to inductively heating a large annular, or ring-shaped workpiece, namely a single-shot (static) process or a scan process.
In a static induction heating process the region of the workpiece that is required to be heat treated can be surrounded by a single-turn or multi-turn induction coil. For example to metallurgically harden a region on the inside diameter 90a of annular workpiece 90 (FIG. 1(a)), an induction coil can be positioned inside of the formed annulus, and alternating current (AC) is supplied to the induction coil to establish a magnetic field around the coil that provides an electromagnetic flux coupling with the inside diameter region of the workpiece for the desired heat treatment. If heat treatment of a region (shown as shaded region 90c in FIG. 1(c)) on the outside diameter 90a′ of workpiece 90, then induction coil 100 can be positioned outside of the formed annulus as shown in FIG. 1(b) and FIG. 1(c). Induction coil 100 is connected to an alternating current power source 102. In this arrangement, induction coil 100 encircles the outer diameter of workpiece 90. The workpiece can be optionally rotated (for example about workpiece central axis A) during the heat treatment process to ensure an even distribution of induced energy around the workpiece's perimeter over the entire heating cycle. Rotation rates are selected to suit process requirements.
When utilizing an encircling induction coil 100 as shown in FIG. 1(a) and FIG. 1(b), the following process parameters play a dominant role in obtaining the required hardness depth, δ, and pattern: frequency of the supplied alternating current; magnitude of the supplied induction power; quenching parameters (such as temperature of the quenchant; quenchant rate of flow (flux density); pressure and concentration of quenchant, for example with aqueous polymer quenchant); and cycle process time. Cycle process time includes: induction heating time; soaking time (if soaking is used); and quenching time. There are two commonly applied methods of quenching in a single-shot heating process of a large annular workpiece. According to one technique as illustrated in FIG. 1(d), upon completion of the induction heating stage, the heated workpiece is positioned within a separate concentric spray quench block (or ring) 104 that is positioned below the inductor 100 and spray-quenched in-place by moving workpiece 90 downwards as shown in FIG. 1(d). Upon sufficient quenching, a surface hardness layer 90c′ will be formed on the surface of the workpiece. In an alternative quenching method as illustrated in FIG. 1(e), the heated annular workpiece 90 is submerged in a quench tank 92 filled with quenchant 94 and quenching takes place inside of the quench tank while the quenchant is usually agitated by suitable means.
One of the main drawbacks of a single-shot heat treatment is the necessity of supplying the induction coil (inductor) with a substantial amount of power since the simultaneous heating method requires a magnitude of power sufficient to raise the temperature of the entire surface of the ring to the required level at required depth. Therefore costly high power induction heating sources are required.
In a scan induction process, an appreciably smaller inductor than that used in the single-shot process, such as short inductor 101 moves in a circular path (concentric with the center of the workpiece) around the outer perimeter of annular workpiece 90 as shown in FIG. 2(a). Single inductor 101 is shown multiple times in FIG. 2(a) and FIG. 2(b) to indicate the directed circular travel path of the inductor, namely from start position A1, followed by sequential (clockwise CW) subsequent quadrant positions B1, C1 and D1. While moving around the workpiece the magnetic flux field established by alternating current flow in inductor 101 couples to a required penetration depth of the workpiece as diagrammatically shown by shaded regions. Single spray quench apparatus 105 moves with (tracks) inductor 101 around the workpiece and is likewise shown multiple times in the figures. Spray quench apparatus 105 may be of suitable form known in the art such as a quench block or jet, and may also be an integral assembly with the inductor. This scan induction process requires significantly less power than the single-shot process since only a small sector of the workpiece is instantaneously flux coupled and inductively heated as inductor 101 moves around the annular workpiece. A disadvantage of this method is the presence of a “soft” zone 90d in the metallurgically hardened (shaded) penetration depth 90c′ as shown in FIG. 2(b) where the workpiece will not be properly heat treated. The soft zone in this example is a function of the length of the coil 101 and its scan speed and is generally in the range of 1 to 9 cm in arc length as shown in FIG. 2(b). The term “soft zone” is used to describe a region where the desired metallurgical heat treatment achieved in the penetration depth elsewhere around the outer perimeter is not achieved. Soft zone 90d is inevitably created due to the tempered region adjoining the final ring section to be heated.
To prevent soft zones while scan hardening without the requirement for an oversized power supply, as required with static one shot hardening, the prior art double inductor/quench apparatus arrangement shown in FIG. 3 can be utilized. A pair of inductors 103a and 103b can be used with each inductor in the pair performing induction hardening for one-half of the annular workpiece 90. In FIG. 3 each inductor surrounds the inner and outer perimeters of the workpiece so that penetration depths into the inner and outer perimeters are heat treated. The arrangement shown in FIG. 3 is further described in “Induction Surface Hardening” by A. D. Demichev, pages 25-26, published by the Leningrad Division of Publishing House “Mashinostryeniye”, Saint Petersburg, RUSSIA, 1979. For simplicity in illustration and description FIG. 4(a) through FIG. 4(c) are provided to describe a double inductor/quench apparatus arrangement where only a penetration depth from the outer perimeter of the workpiece is heat treated. Inductors 103a (counterclockwise) and 103b (clockwise) move in circular counter directions at a constant speed around the outer perimeter of workpiece 90 from starting positions A1 and A2 respectively as shown in FIG. 4(a) through intermediate positions B1 and B2, respectively, as shown in FIG. 4(b), and then to finish positions C1 and C2, as shown respectively, in FIG. 4(c). The counterclockwise arc and clockwise arc from position A1 to position C1 and position A2 to position C2 respectively are less than 180 degrees due to the physical space taken up by both inductors when they are adjacent (side-by-side) to each other at the start and finish positions. Each inductor is supplied the same magnitude of power from a suitable alternating current source through the less than complete semicircular movement around the outer perimeter of the workpiece. As with the single inductor process described above spray quench apparatus 105a and 105b moves with (tracks) inductor 103a and 103b respectively, around the workpiece until the inductors are adjacent to each other at the end of the heating process in positions C1 and C2 as shown in FIG. 4(c). Both spray apparatus are de-energized at these positions and, simultaneously, an auxiliary spray apparatus 105c automatically provides quenchant to the final heat treated sector 90e of the workpiece as shown in FIG. 4(c). The adjacent inductors in the final heating positions C1 and C2 eliminate the presence of soft zones in the final heating positions.
One of shortcomings of the double inductor/spray apparatus process is the difficulty in providing uniform heating, and as a result, a uniform hardness depth 90c in the start and finish positions (A1, A2 and C1, C2). At the start of the heating process, the distance between inductors 103a and 103b can not be immediately adjacent to each other since the magnetic fields established by current flow in each inductor could interfere with each other if supplied by independent power supplies, which can result in lower levels of induced heating.
Additionally after the heating process starts, both inductors 103a and 103b have to travel sufficiently far from each other before quenchant can be supplied from quench apparatus 105a and 105b to heated region 90e of the workpiece 90 as shown in the detail views of FIG. 5(a) and FIG. 5(b). If quenchant is supplied too soon (that is, when the inductors have not traveled sufficiently far apart from each other), quenchant can splash onto heating sectors located under the energized inductors, which results in the formation of unacceptable hardening structures, such as an appearance of regions within the hardness pattern having inappropriate phase transformations, soft spots, and altered microstructures. Therefore, there is always a longer quench delay during the initial induction heating stage compared to the quench delay during scanning.
Both inductors 103a and 103b must travel in opposite directions sufficiently far from each other to avoid quench splashing on the zone being heated as shown in FIG. 5(c) before quench spray 105a′ could begin to be supplied from quench apparatus 105a and 105b. Typically this separation distance can be in the approximate range of 5 to 10 cm. During this unavoidable quench delay time period, there will be heat loss from the previously heated region between inductors 103a and 103b due to a thermal conduction that leads to a heat flow from high temperature regions of the ring towards its cooler regions resulting from a “cold sink effect.” Due to this effect, the previously heated area can cool down to temperatures below the level, and at a rate that is too slow for obtaining a desired fully martensitic structure. During inevitable quench delay, besides the cold sink effect, cooling of the initially heated areas take place due to surface heat losses from thermal radiation and convection. Greater “hardness depth-to-ring thickness” ratios and slower scan speeds of the inductors negatively affect thermal conditions of the initially heated region that is positioned between inductor pair 103a and 103b. A similar difficulty in achieving a desired temperature distribution and hardness profile occurs in the final heating region (positions C1 and C2) of the workpiece as shown in FIG. 4(c) for reasons related to quench delay similar to those described above for the start positions of the inductors.
One object of the present invention is to achieve a metallurgically uniform hardness layer in the region where the induction heating process begins and ends in a two or more inductor/spray apparatus employing a scan heat treatment process for an annular workpiece.